cc/gcc: do not use the core pass-2 to build the baremetal compiler

In case we build a baremetal compiler, use the standard passes:
- core_cc is used to build the C library;
- as such, it is meant to run on build, not host;
- the final compiler is meant to run on host;

As the current final compiler step can not build a baremetal compiler,
call the core backend from the final step.

NB: Currently, newlib is built during the start_files pass, so we have
to have a core compiler by then... Once we can build the baremetal
compiler from the final cc step, then we can move the newlib build to
the proper step, and then get rid of the core pass-1 static compiler...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 diff -durN gcc-4.2.1.orig/libstdc++-v3/configure gcc-4.2.1/libstdc++-v3/configure

 --- gcc-4.2.1.orig/libstdc++-v3/configure	2007-08-03 20:30:21.000000000 +0200

 +++ gcc-4.2.1/libstdc++-v3/configure	2007-08-03 20:33:56.000000000 +0200

 @@ -7514,6 +7514,9 @@

  cat >>conftest.$ac_ext <<_ACEOF

  /* end confdefs.h.  */

  #include <complex.h>

 +#ifdef __UCLIBC__

 +#error ugly hack to make sure configure test fails here for cross until uClibc supports the complex funcs

 +#endif

  int

  main ()

  {